The present invention relates to a biaxially oriented polypropylene film capable of being formed by coextrusion, and characterized by having a relatively high seal strength and a relatively low heat seal initiation temperature. The film of the present invention is well suited for dry food packaging applications.
The world's demand for biaxially oriented polypropylene (BOPP) film has grown by an average of 8.7% per year since 2000. BOPP films comprise at least one propylene-containing layer. Flexible packaging for dry food is one of the major applications of BOPP. At present, dry food packaging is dominated by the multi-ply dry laminated BOPP films. These films are heat sealable and are usually produced by joining a BOPP film with a separate sealant film such as a polyethylene or cast polypropylene (CPP) film through adhesive lamination techniques. Generally, these BOPP laminated structures have heat seal strength of approximately 15-40 N/25 mm and heat seal initiation temperatures (HSIT) around 105° C. (when polyethylene is used as the sealant film).
Coextruded (or “co-ex”) BOPP film is another kind of heat sealable BOPP based film which can be used for packaging. These films are produced by coextrusion of polypropylene and sealant resin to form a sheet and then biaxially stretching the sheet to form the biaxially oriented film. The production of co-ex BOPP is a more cost effective process vs. the adhesive lamination process, as the film is made in a single step and no adhesive is required. However, the heat seal strength of currently available co-ex BOPP films drops to around 5-8 N/25 mm and the HSIT increases by about 10° C. after biaxial stretching. Due to the reduced seal strength, co-ex BOPP films are currently only used for packaging small articles or articles not requiring high seal strengths, such as cigarette packaging, or light-weight food like bread.
It would be desirable to develop co-ex BOPP films to replace the laminated BOPP films in order to cut cost and eliminate the adhesive involved in the lamination process and yet have comparable heat seal strength and HSIT.
Therefore in one aspect of the present invention a multilayer biaxially oriented polypropylene film is provided. The film comprises at least three layers. One layer is an outer layer comprising a first polymer which is a homopolymer polypropylene resin having a melting point of at least 155° C. A second layer is a core layer comprising a blend comprising a second polymer which is a polypropylene homopolymer having a melting point of at least 155° C. and a third polymer having a melting point no greater than 145° C. and a Vicat softening point of at least 60° C., wherein the third polymer comprises no more than 80% by weight of the core layer. A third layer is a sealant layer comprising a fourth polymer having a melting point no greater than 145° C.
Another aspect of the invention is the use of the films of the present invention for packaging dry foods. In particular, the films of the present invention which have a preferred heat seal strength of at least 10 N/25 mm (even when the core layer is less than 10 μm thick), and a preferred heat seal initiation temperature no greater than 130° C., are well suited for this application.